1. Field
The disclosed embodiments generally relate to oscillator circuits. More specifically, the disclosed embodiments relate to a method and an apparatus for starting up a resonant oscillator circuit in a manner which provides inrush protection and mitigates metastability problems.
2. Related Art
Oscillator circuits are commonly used to generate pulses in electrical systems. However, commonly used oscillator circuits can consume a significant about of power, which is a disadvantage for systems that need to conserve power, such as portable computing devices. To solve this problem, “resonant oscillator circuits,” which transfer energy back and forth between inductive and capacitive circuit elements, can be used to generate clock pulses without dissipating a significant amount of power. (For example, see U.S. Pat. No. 5,559,478, entitled “Highly Efficient, Complementary, Resonant Pulse Generation,” by inventor William C. Athas, filed 17 Jul. 1995.)
Unfortunately, existing designs for resonant oscillator circuits are not well suited for certain applications. For example, in one application a resonant oscillator circuit is used to clock the two phases of a switched capacitor block (SCB) in a voltage conversion system. (See patent application Ser. No. 12/535,974, entitled “High-Efficiency Switched-Capacitor Power Conversion,” filed on 5 Jul. 2009 by inventors William C. Athas and P. Jeffrey Ungar which is hereby incorporated by reference). In this application, if the clock signals are first turned on such that power field-effect transistors (FETs) in the SCBs are at or near their minimum on-resistance and the capacitors of the SCB are well below their normal operating voltage levels, the resulting inrush current can damage the FETs. Hence, it is desirable to first clock the SCBs with a low voltage swing, such that the power FETs in the SCB have a high on-resistance, and then gradually decrease the on-resistance by increasing the amplitude of the clock signals.
Another problem is the uncontrolled and unpredictable way in which oscillations commence in a resonant oscillator during power up. As power is applied to the basic two-FET and two-inductor implementation disclosed in U.S. patent application Ser. No. 12/535,974 (cited above), current more or less equally divides between the two inductor branches. Asymmetries in the component values and the layout will eventually cause the two branch currents to become unequal which allows the oscillations to commence. However, the circuit may exist in a balanced, metastable state for an indeterminate period of time. Meanwhile, current builds up in each inductor in proportion to the duration of the metastable state. Consequently, when the circuit eventually exits the metastable state, the initial output pulses can be almost unbounded in magnitude, and may damage downstream components, such as the SCBs, or even the components of the resonant oscillator circuit itself.
Hence, what is needed is a resonant oscillator circuit, which does not suffer from the above-listed problems.